In 3GPP packet networks with GERAN/UTRAN (2G/3G) access, the UE is registered in the Core Network element SGSN via an attach or Routing Area procedure. The SGSN holds a UE context (mobile identity, security parameters) and knows the location of the User Equipment (UE) with Routing Area accuracy. The UE can move freely within the registered RA (Routing Areas), without initiating Routing Area Update procedure, as long as the periodic timer does not expire.
Similarly, in 3GPP packet networks with E-UTRAN (LTE), the UE is registered in the Core Network element MME via an attach or Tracking Area procedure. The MME holds a UE context (mobile identity, security parameters) and knows the location of the UE with a list of Tracking Areas accuracy. The UE can move freely within the registered list of TAs (Tracking Areas), without initiating Tracking Area Update procedure, as long as the periodic timer does not expire.
For E-UTRAN UEs with GERAN or UTRAN capabilities, when the UE reselects between GERAN/UTRAN access and E-UTRAN access, the UE initiates a Routing Area update when leaving EUTRAN access for GERAN/UTRAN in order to register to the SGSN, and the UE initiates a Tracking Area update when leaving GERAN/UTRAN access for EUTRAN access in order to register to the MME.
3GPP has specified a mechanism, called the Idle mode Signaling Reduction (ISR) for reducing the mobility related signaling, i.e. the RAU/TAU procedures when the UE is moving between EUTRAN and GERAN/UTRAN.
At FIG. 1, it is assumed that the ISR function is activated, and thus the UE 2 is simultaneously registered both in a list of Tracking Areas managed by the MME 4 via the EUTRAN Access Network (LTE) 6 and in a Routing Area managed by the SGSN 8 of the GERAN/UTRAN Access Network (2G/3G) 10.
If the ISR function is activated by the network, both MME 4 and SGSN 8 hold a UE context 12 synchronized with each other. The UE 2 can then move freely between the registered list of Tracking Areas (TAs) in EUTRAN and the registered Routing Area (RA) in GERAN/UTRAN 1 without the need to initiate any registration update towards the MME 4 or the SGSN 8.
ISR reduces the mobility-related signalling when the UE reselects in idle-mode between EUTRAN and GERAN/UTRAN registered areas. It achieves signalling load reduction in the network and allows the UE to remain in idle mode and thereby to save battery.
At FIG. 2, it is assumed that the UE is registered in a list of Tracking Areas managed by the MME 4 via the EUTRAN (LTE) Access Network 6 and that the ISR function is not activated by the network. If the UE 2 moves to the target Radio Access Technology (RAT) 2G/3G 10, it shall perform the registration update procedure (Routing Area Update RAU) in the target 2G/3G Network 10, triggering the ISR support negotiation and the UE context synchronization (arrow 16) between the MME 4 and the SGSN 8.
If the ISR function is activated by the target Core Network (CN) node (SGSN 8), the network indicates ‘ISR activated’ to the UE 2 in the registration update (RAU) accept message. The UE 2 is registered in the target Routing Area managed by the SGSN 8 of the 2G/3G Network 10 via the GERAN/UTRAN Access Network and remains also registered in the list of Tracking Areas previously assigned by the MME 4 via the EUTRAN Access Network of the source LTE Network 6. When the ISR is activated by the MME 4, the network indicates ‘ISR activated’ in the TAU accept.
At FIG. 3, it is assumed that the UE 2 is camping in a cell of the source LTE Network 6 and that the ISR function is activated.
If the UE context is modified in the UE 2 and in the Core Network Element of the currently used RAT (the MME 4 of the EUTRAN Access Network 6), i.e. the RAT where the UE is camping, then the UE context in the Core Network Element of the currently not used RAT (the SGSN 8 of the 2G/3G Network 10) becomes unsynchronized.
Such UE context unsynchronization between UE 2, MME 4 and SGSN 8 is caused when a bearer context (PDP context in 2G3G or EPS bearer context in LTE) is added or modified, or when the UE changes a MM context parameter like UE specific DRX parameter or UE Core Network Capability.
If the UE 2 moves back to the target 2G/3G Network 10, the UE context in the SGSN 8 shall be re-synchronized with the UE context of the MME 4 to allow the UE 2 to maintain the established services (packet, security . . . ).
Such special situation where the UE context is modified in one RAT in the UE and the CN node of this RAT, causing the unsynchronization of the UE context in the CN node of the other RAT, is resolved in the current 3GPP specification with the ISR deactivation locally in the UE, generating a need for registration update at next IRAT change, in order to guarantee that the CN node in the other RAT is updated at the next Inter-RAT (IRAT) change via a UE context resynchronization.
In order to force this resynchronization, the UE 2 locally deactivates the ISR procedure in the RAT (the EUTRAN 6 in the example of FIG. 2) where the UE context is modified so that a registration update procedure is forcibly initiated when the UE 2 returns to the target RAT (the GERAN/UTRAN 10). During the registration update procedure, the UE context of the source RAT MME 4 is transferred to the target RAT SGSN 8.
In idle mode, when the UE 2 reselects between the source RAT and the target RAT, the UE context transfer from source RAT CN node to target RAT CN node is performed during the registration update procedure in the target RAT initiated by the UE 2.
As described in 3GPP 23.401 5.3.3 (TAU procedures), upon TAU triggered by inter-RAT cell reselection by the UE, the target RAT Core Network (CN) node fetches the UE context and ISR support from the source RAT CN node, using Context Request message, and, once received in Context Response message, it provides the ISR activated status to the source RAT CN node in Context Acknowledge message.
3GPP has initially designed the signaling reduction for idle-mode when the UE 2 moves in idle mode and reselects between E-UTRAN and GERAN/UTRAN cells within the registered areas. 3GPP has then extended the ISR procedures of the idle-mode to the connected mode.
Indeed, as defined by 3GPP 23.401 4.6.3.2 (ECM-CONNECTED mode), UEs in connected mode initiates the TAU procedure in a registered tracking area only if TIN is set to ‘PTMSI’, i.e. ISR is deactivated, similarly to the idle mode behavior.
This extension enables the UE 2 to omit the TAU/RAU in connected mode when ISR is activated and UE 2 is handovered within the registered areas.
Therefore the signaling reduction illustrated in FIG. 1 for inter-RAT cell reselection in idle mode also applies in case of inter-RAT handover.
Moreover, as defined by 3GPP 23.401 5.5.2 (Inter-RAT handover), the ISR activation negotiation and UE context synchronization between the source RAT CN node and the target RAT CN node are performed during Inter-RAT handovers (FIG. 4), similarly to the idle-mode where it is performed during the TAU/RAU procedures initiated by UE 2 (FIG. 2).
During the inter-RAT handover initiated by the network (FIG. 4), as described in 3GPP 23.401 5.5.2 (Inter-RAT handover), the UE context and ISR support is pushed by the source RAT CN node to the target RAT CN node using Forward Relocation Request message, and the target RAT CN node returns the ISR activated status to the source RAT CN node in Forward Relocation Complete message. If the UE 2 performs a registration update procedure in the target RAT (e.g. UE moves outside the registered area in the target RAT), a reduced registration update without the UE context transfer between the CN nodes occurs in the network, because they already occurred during the handover procedure.
At FIG. 4, it is assumed that the UE 2 moves from the LTE Network 6 (source RAT) to the 2G/3G Network 10 (target RAT) via an inter-RAT (IRAT) handover. In this case, the UE context 12 stored in MME 4 is transferred to the SGSN 8 during the handover procedure. After the inter-RAT handover, the UE contexts stored in the SGSN 8 and MME 4 are therefore resynchronized in the network. Then, the registration update in the target RAT after the handover procedure is performed if the ISR function was not activated by network, or if the UE 2 deactivated the ISR procedure locally, or if a regular trigger in the target RAT occurred (e.g. new area not yet registered . . . ).
Primarily designed for reducing mobility signaling at inter-system change in idle-mode, the ISR feature also achieves signaling reduction at inter-system handover. As per 3GPP 23.401, while the UE camps in one RAT, the source RAT, the modification of the UE context (bearer context, DRX specific parameter, UE Core Network capabilities) causes unsynchronized state information between UE, SGSN and MME. This special situation is resolved with the ISR deactivation locally in the UE in order to guarantee that the CN node in the source RAT transfers the latest UE context to the CN node of the target RAT, in the Context Response message, during the registration update procedure initiated by the UE at the next IRAT cell reselection.
However, when the inter-RAT change is a handover, the CN node from the source RAT already transfers the UE context to the CN node of the target RAT in Forward Relocation Request message during the handover procedure.
Therefore, when the ISR function is activated by the network and locally deactivated in the UE 2 only as a result of a UE context modification (arrow 20) in source RAT as illustrated at FIG. 5, the UE 2 performs the registration update (arrow 22) when moving to the 2G/3G Network 10 and thus the signaling is not reduced.
In such situation, this registration update is unnecessary when the IRAT change is a handover and the target cell belongs to a previously registered area because the CN nodes MME and SGSN are unchanged, and the target CN node SGSN 8 already received the UE context from source CN node MME 4 in Forward Relocation Request message during the handover procedure.
Indeed, contrary to the idle mode where the registration update in the target RAT is required to resynchronize the CN node in the target RAT when ISR was locally deactivated in the source RAT due to UE context modification (FIG. 3), the UE context in the connected mode case is already resynchronized during the handover procedure (FIG. 4) before any registration update is initiated. Thus the registration update procedure is not required for UE context synchronization purpose when the UE is handovered to a previously registered area and ISR was deactivated in the source RAT in the UE for the only reason of updating the UE context in the CN node of the target RAT at next IRAT change.
Numerous triggers for ISR local deactivation have been defined by 3GPP, e.g. bearer context modification, change of DRX parameter, change of UE CN capabilities and thus, the efficiency of the signaling reduction targeted by ISR is decreased.
The invention aims at improving the efficiency of signaling reduction in case of inter-RAT handover, when ISR is activated by the network and the triggering events of the ISR deactivation in the source RAT generating a need for registration update at next IRAT change only correspond to a UE context modification in the source RAT and when the UE is handovered back from a source area covered by a source Telecommunication Network and to an already registered target area covered by a target Telecommunication Network.